The invention is related generally to screw drive mechanisms, and more particularly, to a lead screw engagement system.
The infusion of fluids such as parenteral fluids into the human body is accomplished in many cases by means of a syringe pump having a lead screw on which a screw drive mechanism is mounted. Rotation of the lead screw is translated into linear motion by the screw drive mechanism and is transmitted to the syringe plunger by a plunger driver. The plunger driver is typically rigidly connected to the screw drive mechanism. A split nut or half nut is used as part of the screw drive mechanism in some pumps for easy and rapid repositioning of the screw drive mechanism and plunger driver on the lead screw to engage a new syringe plunger. An activating lever or other mechanical interface may be provided at the plunger driver to separate the split nut from the lead screw threads and to disengage the plunger driver from the present syringe plunger. The plunger driver is retracted and the syringe replaced. The operator uses the activating lever or other mechanical interface to disengage the split nut from the lead screw, moves the plunger driver and split nut along the lead screw to the appropriate position and inserts the new syringe plunger. The operator then releases the activating lever to clamp the plunger driver onto the new plunger and to engage the lead screw threads with the split nut. Unfortunately, re-engagement with the lead screw threads does not always occur at this time due to the shape of the thread profile.
As shown in FIG. 1, in some cases the crests of the threads 10 of the split nut align with and contact the crests of the threads 12 of the lead screw rather than the threads engaging each other. Because these threads may be relatively flat at their crests, they do not engage each other until the lead screw has turned a sufficient amount and the bias device of the screw drive mechanism has forced the threads of the split nut to drop into or engage the lead screw threads. Under low infusion rates, such sufficient lead screw rotation may take a substantial amount of time, this time then being a period during which the patient does not receive the infusion fluid.
In another case as presented in FIG. 2, the non-driving surfaces of the threads 10 of the split nut may contact the non-driving surfaces of the threads 12 of the lead screw when the activating lever is released. The arrow indicates the direction of forward movement of the split nut and plunger driver. Due to the thread clearance 14 between the two sets of threads, some amount of time may pass before the driving surfaces of the lead screw threads 12 contact the driving surfaces of the split nut threads 10 and actually begin to move the split nut and plunger driver. This also would be a time period during which the patient receives no infusion fluids.
In yet another case as presented in FIG. 3, the threads 10 of the split nut may have only partially engaged the threads 12 of the lead screw. If this should happen on the driving surfaces of the threads of the lead screw as shown, the threads 10 of the split-nut may not drop into the threads 12 of the lead screw because of the large amount of back pressure provided by the full syringe and because of the force vector developed by the lead screw threads which tends to force the split nut threads 10 up and out of the lead screw threads 12. If the biasing force on the split nut is not strong enough to overcome all of these factors, the split nut threads 10 may be forced out of contact with the driving surfaces of the threads 12 of the lead screw and into the position shown in FIG. 1. This case would likewise result in a time period during which the patient would not receive the infusion fluid and this time period may exceed that associated with FIG. 1.
In certain prior systems having a detector to indicate thread engagement between the lead screw and the screw drive mechanism, the case shown in FIG. 3 may be indicated as thread engagement. In many of these prior systems, the detector monitors the position of the lever used by the operator at the plunger driver rather than the engagement mechanism at the lead screw itself. And in some cases, these levers are coupled to the split nut through a spring rather than through a rigid mechanical connection. Where the lever is in the correct position for engagement of the screw drive mechanism with the lead screw but the spring coupling has not transmitted the lever force to the split nut, an inaccruate indication of screw engagement may be given with a resulting time period during which the patient receives no infusion fluid.
Hence, those skilled in the art have recognized a need for an lead screw engagement system in which the screw drive mechanism and lead screw are made to engage as soon as possible so that no delay in fluid infusion occurs. Further, a need for a more accurate and reliable indicator of engagement has been recognized. The invention fulfills these needs.